I got it from a German Bricklink store.
(I'm not sure if I can post direct links to BL stores here, so I won't)Every now and then, parts in special colors appear on Bricklink; so was this green bandana.

The "minifig hat rag" came in green in Forestman's Crossing, and is somewhat rare. However, as far as I know, the bandana around his neck is not from any set. We've seen a lot of odd color minifig accessories lately that seem to be official molds but don't seem to come from any sets. I'm actually waiting on a Bricklink order with a black beard myself.